Network overlay location system and method for air interface with frequency stopping

ABSTRACT

Embodiments of a system and method are disclosed that enable geo-location of a mobile appliance ( 20 ) communicating over a frequency hopping air interface for a network overlay geolocation system having plural wireless location sensors ( 30 ). The wireless location sensors ( 30 ) include a radio receiver channel capable of receiving the forward channels of the air interface transmitted from the base station ( 10 ) to the mobile appliance ( 20 ). The network overlay geo-location system of the present disclosure monitors these forward channels and measures parameters that allow the geolocation system to synchronize with the hopping of the air interface on the reverse channel. The plurality of sensors measure an attribute of a signal on the reverse channel to thereby enable geo-location. Synchronization can also be reference to a stable system clock and the plurality of sensors may be tuned in reference to the system clock to enable measurement of the frequency hopping reverse channel.

CROSS REFERENCES

The present application is co-pending with and claims priority benefitof provisional application Ser. No. 60/418,342, entitled “Geolocation ofMobile Appliances”, filed on Oct. 16, 2002, the entirety of which ishereby incorporated herein by reference.

The present application is related to and concurrently filed withapplications titled “A NETWORK OVERLAY GEO-LOCATION SYSTEM WITH SMARTANTENNAS AND METHOD OF OPERATION” Ser. No. #, “WIRELESS COMMUNICATIONNETWORK MEASUREMENT DATA COLLECTION USING INFRASTRUCTURE OVERLAY-BASEDHANDSET LOCATION SYSTEMS” Ser. No. #, “A SYSTEM AND METHOD FORESTIMATING THE MULTI-PATH DELAYS IN A SIGNAL USING A SPATIALLY BLINDANTENNA ARRAY” Ser. No. #; “A SYSTEM AND METHOD FOR ENHANCING THEACCURACY OF A LOCATION ESTIMATE, Ser. No. #; and “SYSTEM AND METHOD FOROPERATING A NETWORK OVERLAY GEO-LOCATION SYSTEM WITH REPEATERS” Ser. No.#, filed Oct. 16, 2003, the entirety of each of these applications isincorporated herein by reference.

BACKGROUND

This disclosure is directed to a wireless communications network overlayfor determining the location of mobile appliances.

The use of wireless communication devices such as telephones, pagers,personal digital assistants, laptop computers, etc., hereinafterreferred to collectively as “mobile appliances”, has become prevalent intoday's society. Recently, at the urging of public safety groups andothers, there has been increased interest in technology which candetermine the geographic position, or “geo-locate” a mobile appliance incertain circumstances. For example, the Federal Communication Commission(FCC) has issued a geo-location mandate for providers of wirelesstelephone communication services that puts in place a schedule and anaccuracy standard under which the providers of wireless communicationsmust implement geo-location technology for wireless telephones when usedto make a 911 emergency telephone call (FCC 94-102 E911).

In addition to E911 emergency related issues, wirelesstelecommunications providers are developing location-enabled servicesfor their subscribers including roadside assistance, turn-by-turndriving directions, concierge services, location-specific billing ratesand location-specific advertising.

In a network-based geo-location system, the mobile appliance to belocated is typically identified and radio channel assignments determinedby (a) monitoring the control information transmitted on a radio channelor a wireline interface within the wireless network for telephone callsbeing placed by a mobile appliance to thereby detect calls of interest,e.g., 911 calls, or (b) a location request provided by a non-mobileappliance source, e.g., an enhanced services provider. Once a mobileappliance to be located has been identified and radio channelassignments determined, the geo-location determining system is firsttasked to determine the geo-location of the mobile appliance and thendirected to report the determined position to the requesting entity orenhanced services provider.

The monitoring of the RF transmissions from the mobile appliance or froma wireline interface to identify calls of interest is known as “tipping”and generally involves recognizing a call of interest being made from amobile appliance and collecting the call setup information. Once themobile appliance is identified and the call setup information iscollected, the location determining system can be tasked to geo-locatethe mobile appliance.

FIG. 1 shows a conventional mobile-appliance communication system havinga mobile switch controller 45 connected to base stations 10 forcommunicating with a mobile appliance 20. Each base station 10 containssignal processing equipment and an antenna for transmitting to andreceiving signals from the mobile appliance as well as other basestations and centrally located control and processing stations. A mobileappliance location determining sensor 30, sometimes referred to as awireless location sensor (“WLS”), may be positioned at some or all ofthe base stations 10 to determine the location of a mobile appliancewithin the signal coverage area of the communication system. The antennamay be a multi-element antenna. The Mobile Positioning Center 40 (MPC)generally tasks the location system to locate a mobile appliance, andsupplies channel assignment information to the location system via theGCS 50.

A network overlay system is generally composed of two main components,one component which resides at the base station that makes measurementson an RF signal emanating from a wireless device, known in the art as ageo-location sensor or a wireless location sensor (“WLS”) 30, and theother component which resides at a mobile switch that tasks thegeo-location sensor groups to collect data and then uses the data tocompute a location estimate. This component is generally referred to asthe Geo-location Control System 50 (“GCS”).

As discussed above, one way to initiate a geo-location evolution is foran outside entity to task the GCS to generate a location estimate on aparticular (target) mobile appliance. The tasking may be accompanied byinformation on the mobile of interest including the serving base stationand sector for the call as well as the RF channel being used by thewireless communications network to complete the wireless connection.Typical examples of information that must be known include the RFfrequency especially where Frequency Division Multiple Access (“FDMA”)is used, the time slot of the transmission for Time Division MultipleAccess (“TDMA”) systems, and the spreading code for Code DivisionMultiple Access (“CDMA”) systems. In general, this information is usedto tune the radio receivers of the WLSs so that measurements related tothe geo-location estimation can be made. Once the GCS receives thegeo-location tasking, the GCS tasks a set of WLS units to takemeasurements of the RF emission of the target mobile. The WLS units takethe measurements and report the data from the measurements to the GCS.The GCS then computes a location estimate using a mathematical or datamatching algorithm, as is well known in the art.

Alternatively, a geo-location evolution may commence upon detection of acall of interest, such as a 911 call. In this scenario, control channelsused to set up calls in the wireless network can be scanned to detectthe placement of a call of interest. The signaling that occurs on thecontrol channel can be used to determine location if the control data iscontained on an RF channel, or alternatively the RF traffic channelparameters can be extracted from the control channel messaging todetermine which traffic channel to use for geo-location relatedmeasurements.

In operation, these network overlay location systems typically takemeasurements on RF transmissions from mobile appliances at base stationlocations surrounding the mobile appliance and estimate the location ofthe mobile appliance with respect to the base stations. Because thegeographic location of the base stations is known, the determination ofthe location of the mobile appliance with respect to the base stationpermits the geographic location of the mobile appliance to bedetermined. The RF measurements of the transmitted signal at the basestations may include the time of arrival (“TOA”), time difference ofarrival (“TDOA”), the angle of arrival (“AOA”), the frequency differenceof arrival (“FDOA”), the signal power, or the unique/repeatable radiopropagation path (radio fingerprinting) derivable features. In addition,the geo-location systems can also use collateral information, e.g.,information other than that derived for the RF measurement to assist inthe geo-location of the mobile appliance. Such collateral informationmay include the location of roads, dead-reckoning, topography, mapmatching etc.

Network overlay location systems typically locate a mobile appliance onthe traffic channels of a wireless network. Network overlay systemstypically use sensors employing techniques of TDOA supplemented with AOAto perform a multi-site location computation. The traffic channelassignment information is provided through a separate process, with oneoption being a wire line interface 41 (see FIG. 1) providing MOBINFO(IS-41 mobile information) parameters passed by the Mobile PositioningCenter (“MPC”) 40 as part of the GPOSREQ (J-STD-036 Geolocation PositionRequest) message.

In general, the overlay system tunes radio receivers of the WLS to an RFchannel occupied by the mobile appliance of interest to make thesemeasurements. As noted above, information about the channel must beknown for the radio receivers in the sensors to be tuned. One class ofair interfaces that cannot be located with the previously definedinformation used in the prior art are frequency hopping interfaces.Frequency hopping signals change RF frequency rapidly over time. Forexample a frequency sequence {right arrow over (f)} is comprised of asequence of N different frequencies {right arrow over (f)}=(f₀, f₁, f₂,f₃ . . . f_(N-1)) with each hop typically having a duration T. Frequencyhopping, as is well known in the art, is used in mobile radioapplications to mitigate the effects of stationary interference ormulti-path that may exist if the transmission is on a certain RFfrequency. By rapidly changing frequencies, the probability of prolongedinterference or fading due to multi-path is reduced.

In general the hopping sequences and duration of each hop aredeterministic and known by both the mobile appliance and the servingbase station. However, even if the hopping sequence is known, the radioreceivers in the geo-location system cannot know where in the sequenceto look for the mobile appliance signal, and location estimatesdetermined from measurements made without this information can be madeon the wrong mobile appliance since a signal from other than the targetmobile appliance may hop into the RF channel being measured.

In communications between the base station and mobile appliance, themobile's frequency hopping transmission on the reverse channel will besynchronized in some manner to the forward channel transmission as isknown in the prior art. This synchronization is achieved for some airinterfaces by having the mobile appliance use the reception and timingof broadcast control frames from the base station to establish a timinggrid for hopping its transmission RF frequency. By having the timinggrid, and being told what hopping pattern to use including the first RFfrequency in the pattern, the base station and mobile appliance can knowwhat frequency each will transmit on in time. Further refinement of thehopping timing is done by having the base station advance transmissionin time to compensate for range uncertainties from the base station.This is accomplished by having the base station send timing advance (TA)control data to the mobile on forward transmission channels while duringthe course of traffic transmission. This synchronization between theforward (typically taken to be from the base station to the mobileappliance) and reverse (typically taken to be from the mobile applianceto the base station) channels is required for mobile appliance and basestation communications. The forward channel transmission may or may notbe synchronized to any other reference at the serving base station oranother base station. When it is synchronized, sources ofsynchronization include GPS time, and timing derived from communicationsfacilities used by the base station such as T1 or E1 timing.

The subject matter of the present disclosure obviates the deficienciesin the prior art by providing a system and method for the radioreceivers of the wireless location sensors to be tuned to follow thehopping sequence to facilitate measurement of a signal from a targetmobile appliance that is using a frequency hopping air interface.

It is an object of the disclosure to present a novel system and methodfor geo-locating a frequency hopping mobile appliance. The novel systemand method may monitor communication between a base station and a targetmobile appliance on a forward channel, retrieve synchronizationinformation from the forward channel; and synchronize the wirelesslocation sensors with the mobile appliance on the reverse channel. Themeasurements of the location system may be made by a sensor located atone base station or other site on the forward and reverse transmissionsfrom wireless communications system equipment co-located at that site,or from wireless communications equipment transmissions located atneighbor sites. Thus timing synchronization and other informationnecessary to follow frequency hopping patterns on forward and reversetransmissions can be accomplished without having location system sensorsat each wireless communications system base station site. The system andmethod may also include measuring an attribute of the mobile appliance'ssignal to thereby generate an estimate of the geo-location of the mobileappliance based at least in part upon the measured attribute.

In is also an object of the disclosure to present a novel improvedmethod for geo-location of a wireless appliance including the steps ofretrieving channel assignment information from an MPC or by monitoringan RF or wireline link for channel assignment information, monitoringthe reverse channel at a plurality of sensors for the wirelessappliance's signal, measuring an attribute of the signal at theplurality of sensors, and determining the location of the wirelessappliance from the measured attributes. The improved method may alsoinclude monitoring a forward link of the mobile appliance and retrievingsynchronization information from the forward link to thus enablemeasuring an attribute of the signal and for geo-locating the mobileappliance.

It is still another object of the disclosure to present, in a wirelesscommunication system communicating with a frequency hopping mobileappliance with a determinable hopping frequency and duration, a novelnetwork overlay geo-location system for geo-locating the mobileappliance. The geo-location system may include a plurality of sensorslocated at the plural base stations for measuring an attribute of areverse communication channel, a geo-location system controller operablyconnected to each of the plurality of sensors, and an informationchannel for transmitting information regarding the mobile appliance'sfrequency hopping characteristics to the plural wireless locationsensors. In the system, the forward channel may contain synchronizationinformation for tuning the plurality of sensors to the reversecommunication channel, thus allowing the sensors to collect measurementsto geo-locate the mobile appliance.

It is another object of the disclosure to present a novel method forgeo-location of the wireless appliance including the steps of monitoringthe forward channel for synchronization information and using thesynchronization information to tune the plurality of sensors to thereverse channel.

These and other advantages of the disclosed subject matter over theprior art will be readily apparent to one skilled in the art to whichthe disclosure pertains from a perusal or the claims, the appendeddrawings, and the following detailed description of the preferredembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a standard network overlay geo-locationsystem with a host wireless communication system.

FIG. 2 is a flow chart illustrating an embodiment of the disclosedsubject matter.

FIG. 3 is a flow chart illustrating an additional embodiment of thedisclose subject matter.

DETAILED DESCRIPTION

To enable geo-location of a mobile appliance communicating over afrequency hopping air interface, the radio receivers of the WLSs must besynchronized with the reverse channel. To accomplish this, a radioreceiver channel is created in the network overlay equipment that iscapable of receiving the forward channels of the air interfacetransmitted from the base station to the mobile appliance. The radioreceiver channel can be dedicated or a shared resource. The radioreceiver channel will preferably be employed at each base station and/orWLS. Network overlay geo-location systems in the prior art do notreceive these forward channels as they do not provide any direct sourceof measurement data from which to estimate the location of the mobileappliance. The network overlay location system of the present disclosuremonitors these forward channels to measure parameters that allow thegeo-location system to synchronize with the hopping of the airinterface, in the same manner that facilitates communication between amobile appliance and a base station. These parameters may include, butare not limited to, the phase of the hopping cycle (i.e., the points intime when the mobile's RF transmission changes frequency) and thelocation in the sequence of hopping frequencies where the reversechannel is currently operating, sometimes referred to as hoppingsequence position. These parameters can be obtained from the HoppingSequence Number (HSN) with references which algorithm to use to choose afrequency in the list of allowable frequencies for any given frame, theMobile Allocation list (MA) which is the list of frequencies that can behopped to, and the Mobile Allocation Index Offset (MAIO) which is a listof assigned offset into the MA list that sets which frequency thehopping starts with for each mobile.

Measurements of the forward channel parameters coupled with knowninformation about the hopping sequence, such as the duration of thehops, the sequence of hopping frequencies and the particularsynchronization of the reverse channel hopping sequence to the forwardchannel hopping sequence, allow the WLSs in the network overlay locationsystem to receive the reverse channel transmitted from the mobileappliance, remain in synchronization with each frequency hop of thereverse channel, and thereby perform geo-location measurements of themobile appliance. This is possible since the hopping sequence of theforward channel is synchronized with the hopping sequence of the reversechannel in a known fashion. Furthermore, the hopping sequence of theforward channel may or may not be synchronized to any reference at thebase station or any other base station. Therefore, in order to obtainthe hopping sequence of the forward channel, transmitted signals on theforward channel must be received and information gathered from there asdescribed herein.

It may also be advantageous to maintain synchronization with the reversechannel hopping signal without constantly monitoring the forwardchannels. This can be accomplished by referencing the determined hoppingphase and position in the sequence to a stable time base availablewithin the network overlay location system or within the wirelesscommunication system. An embodiment may use Global Positioning System(“GPS”) time for this purpose. Currently, most GSM base stations are notsynchronized to each other, so the radio receiver channel is required tomeasure the forward channels for each base station providing service tomobile appliances. If the base stations are synchronized, this would notbe required, and fewer measurements could be made. Without base stationsynchronization or common reference usage, and network overlay equipmentusing a common reference, measurement devices at different base stationscan make measurement on a mobile appliance even if only one of themeasurement devices has synchronized to the forward channel associatedwith the base station serving the mobile appliance.

The parameters measured from the forward channel to detect hopping phaseand position in the sequence may include specific informationtransmitted as control information or may be transmission relatedparameters.

FIG. 2 shows a representative flow chart for an embodiment to enablegeo-location of mobile appliances employing frequency hoppingcommunication scheme 200.

In block 201 the GCS receives tipping information including the targetmobile's channel information. This information may include hoppingsequence and hop duration. The geo-location system monitors the forwardchannel from the serving base station to the target mobile in order toextract synchronization information in block 203. This step may takeplace in sequence, or occur prior to or in parallel with other steps inthe process. The GCS then tasks WLSs proximate to the target mobile'sserving sector to tune to the target mobile's reverse channel in block205. As discussed above, tuning to the reverse channel may include usingthe synchronization information retrieved from the forward channel, thehopping sequence, and the hop duration. Monitoring the forward channeland tuning the WLS to the reverse channel may be accomplishedcontinuously in order to maintain synch. It is also contemplated thatonce the synchronization information is taken from the forward channel,continuous monitoring, or any monitoring, of the forward channel may nolonger be necessary or desired. The WLS then measures a parameter of thetarget mobile's signal in block 207. The measurements are provided tothe GCS to determine a geo-location estimate for the target mobile usingthe measured parameters as shown in block 209.

Alternatively as shown in FIG. 3, is a method 300 where the hoppingphase and position in the sequence can be referenced to a stable timebase such as, but not limited to, a GPS clock maintained by the system.In this method the GCS receives tipping information including the targetmobile's channel information in block 201. The geo-location systemmonitors the forward channel from the serving base station to the targetmobile in order to extract synchronization information in block 202. TheGCS system then references the determined hopping phase and position inthe sequence to the system clock in block 304. The GCS then tasks WLSsproximate to the target mobile's serving sector to tune to the mobile'sreverse channel using the system clock 308 in block 306. The systemclock being one or both of the network overlay clock and the GSM clock.Since the position and phase of the hopping sequence are referenced tothe system clock, monitoring the forward channel is no longer requiredto maintain synch. The WLSs then measure a parameter of the targetmobile's signal in block 207. The measurements are provided to the GCSto determine a geo-location estimate for the target mobile using themeasured parameters as shown in block 209.

While preferred embodiments of the present invention have beendescribed, it is to be understood that the embodiments described areillustrative only and the scope of the invention is to be defined solelyby the appended claims when accorded a full range of equivalents, manyvariations and modifications naturally occurring to those skilled in theart from a perusal hereof.

1. A method for generating an estimate of the geo-location of afrequency hopping mobile appliance operating within a wirelesscommunication system with a plurality of base stations and having anetwork overlay geo-location system with a plurality of wirelesslocation sensors, comprising the steps of: monitoring at the wirelesslocation sensors a signal on a forward channel between one of theplurality of base stations and the mobile appliance; retrieving at saidwireless location sensors synchronization information from the forwardchannel; synchronizing said wireless location sensors with a reversechannel between the mobile appliance and the base station as a functionof the synchronization information from the forward channel to therebyreceive at said wireless location sensors a signal on the reversechannel; measuring at said wireless location sensors an attribute of thereverse channel signal; and, generating an estimate of the geo-locationof the mobile appliance based in part upon the measured attribute. 2.The method of claim 1 wherein the step of monitoring is accomplished bya dedicated receiver at said wireless location sensors.
 3. The method ofclaim 1 further comprising the step of receiving in said wirelesslocation system channel assignment information including hoppingsequence.
 4. The method of claim 1 wherein the synchronizationinformation comprises hopping sequence position.
 5. The method of claim1 wherein the synchronization information comprises hopping sequencephase information.
 6. The method of claim 1 further comprising the stepof referencing the synchronization information with a network overlayclock.
 7. The method of claim 6 further comprising the step of changingthe monitoring frequency of the plurality of wireless location sensorsbased at least in part on the network overlay clock.
 8. The method ofclaim 1 wherein the plurality of wireless location sensors changemonitoring frequency based in part on the synchronization information.9. In a method for geo-locating a mobile appliance comprising the stepsof retrieving channel assignment information from a geolocation controlsystem, monitoring a reverse channel at a plurality of sensors for asignal from the mobile appliance, measuring an attribute of the reversechannel signal at the plurality of sensors, and determining the locationof the wireless appliance from the measured reverse channel signalattributes, the improvement comprising the steps of: monitoring a signalin the forward channel to the mobile appliance; retrievingsynchronization information from the forward channel signal; determiningsynchronization information for the reverse channel from thesynchronization information retrieved from the forward channel;monitoring the reverse channel as a function of the determinedsynchronization information; and, measuring an attribute of a signal inthe reverse channel from the mobile appliance to thereby geo-locate themobile appliance.
 10. The method of claim 9 wherein the forward channelis a frequency hopping channel.
 11. The method of claim 10 wherein thereverse channel is a frequency hopping channel.
 12. The method of claim11 including the step of receiving channel assignment informationincluding hopping sequence and hop duration.
 13. The method of claim 11wherein the synchronization information for the reverse channelcomprises hopping sequence position.
 14. The method of claim 11 whereinthe synchronization information for the reverse channel comprises phaseinformation.
 15. The method of claim 11 further comprising the step ofreferencing the synchronization information with a network overlayclock.
 16. The method of claim 15 further comprising the step ofchanging the monitoring frequency of the plurality of sensors based onthe network overlay clock.
 17. The method of claim 11 wherein theplurality of sensors change monitoring frequency based in part on thesynchronization information.
 18. A system for generating an estimate ofthe geo-location of a frequency hopping mobile appliance operatingwithin a wireless communication system with a plurality of base stationsand having a network overlay geo-location system, comprising: pluralwireless location sensors for: monitoring a signal on a forward channelbetween one of said plurality of base stations and the mobile appliance;retrieving synchronization information from said forward channel;synchronizing with a reverse channel between the mobile appliance andthe base station as a function of the synchronization information fromthe forward channel to thereby receive a signal on the reverse channel;and measuring an attribute of the reverse channel signal; a geolocationcontrol system operably connected to each of said plural wirelesslocation sensors for generating an estimate of the geo-location of themobile appliance based in part upon the measured attribute from saidplural wireless location sensors.
 19. The system of claim 18 furthercomprising circuitry for providing a stable time reference.
 20. Thesystem of claim 19 wherein said circuitry is operably connected to eachof said plurality of sensors.
 21. The system of claim 20 wherein saidplurality of sensors are tuned to the reverse communication channelbetween the mobile appliance and one of the plural base station usingsaid stable time reference.
 22. The system of claim 19 wherein saidcircuitry is a global positioning system clock.
 23. In a wirelesscommunication system with plural base stations and a network overlaygeo-location system with a plurality of sensors wherein at least one ofthe plural base stations communicates with a wireless appliance over aforward channel and the wireless appliance communicates with the one ofthe plural base stations over a reverse channel the reverse channelbeing a frequency hopping channel, and wherein the forward channel andcontains information to synchronize the base station with a hoppingsequence of the mobile appliance over the reverse channel, a method ofgeo-location of the wireless appliance comprising the steps ofmonitoring the forward channel for synchronization information andtuning the plurality of sensors to the reverse channel with thesynchronization information.
 24. The method of claim 23 furthercomprising the step of referencing the synchronization information to asystem clock.
 25. The method of claim 24 wherein the plurality ofsensors are tuned to the reverse channel using the system clock.
 26. Themethod of claim 25 wherein the system clock is a global positioningsystem clock.
 27. The method of claim 6 further comprising the step ofchanging the monitoring frequency of the plurality of wireless locationsensors based at least in part on a GSM system clock.